The invention relates to a method of regenerating sand regained from sand molds and/or sand cores which are produced from the sand and an inorganic binder, for example, soluble glass, and are used in metal casting.
In metal casting, sand cores solidified by means of a soluble glass binder are used, for example, for forming cavities. For this purpose, large quantities of sand are required, for example, for the series production of automobiles.
Particularly for reasons of cost and environmental protection, used sand, i.e. sand regained from sand cores, is used for the sand core production instead of new sand.
A thermal regeneration is sufficient in the case of a use of used sand bound by means of an organic binder. In the case of used sand bound by means of inorganic binders, a thermal regeneration will not be sufficient. Here, the regeneration usually takes place in two stages in a first mechanical and a second thermal regeneration stage. During the mechanical regeneration, a large fraction of the inorganic binder is removed from the surface of the sand grains. However, chemically active remnants of inorganic binder remain. These chemically active binder remnants are melted during the thermal regeneration, envelop the sand grains with a thin layer, subsequently cool down again and are thereby largely thermally deactivated. The regenerated used sand is then recycled. After the regeneration of the used sand, a percentage of new sand will be added.
By means of the regeneration, binder remnants are removed from the used sand. A binder remnant present in the used sand will have an effect on the sand core and sand mold production. As a result, the processing characteristics of the molding material, thus, of the mixture of sand and soluble glass binder, can, for example, change the flowability of the molding material and the setting rate. In addition, the firmness of the sand core and of the sand mold may be impaired by binder remnants in the used sand.
However, by means of the regeneration of the sand by a mechanical and a thermal regeneration stage, the binder remnants on the surface of the sand grains are only incompletely removed. This context is becoming more significant in the case of a closed material cycle with multiple regenerations of the used sand because a multiple regeneration of the used sand results in a corresponding accumulation of binder remnants.
It is an object of the invention to improve the regeneration of the used sand, which is obtained from sand cores or molds solidified by use of inorganic binders, such that the binder in the used sand is eliminated as completely as possible or a remaining remnant of the binder is at least chemically deactivated.
According to the invention, this and other objects are achieved by a method of regenerating sand obtained from sand molds and/or sand cores which are produced from the sand and soluble glass as a binder and are used in metal casting. The obtained said is subjected to a mechanical regeneration stage and a thermal regeneration stage and is returned in a cycle to the sand mold and/or sand core production. The sand is mixed with a hardener for soluble glass during the mechanical regeneration stage or after the mechanical and before the thermal regeneration stage.
According to the invention, the used sand, thus the sand obtained from the sand molds and/or sand cores, is grain-singled in the next mechanical regeneration stage, for example, by use of a crusher. Subsequently, the binder can be removed by rubbing the sand grains against one another, for example, by way of a mechanical-pneumatic treatment.
After the mechanical regeneration stage, a binder remnant remains in the used sand. This binder remnant consists of an active fraction, which dissolves under sand core or sand mold production conditions, and of an inactive fraction, which is insoluble under sand core or sand mold production conditions. While the active fraction impairs the processing characteristics of the molding material and the firmness of the sand core or of the sand mold, the inactive fraction has no influence on the processing characteristics of the molding material as well as the firmness of the sand core and of the sand mold.
In order to eliminate the active binder fraction, according to the invention, the grain-singled sand is mixed during or after the mechanical regeneration stage with a hardener for soluble glass, whereupon it is subjected to a thermal reaction step.
An agent for the cross-linkage or polymerization of the silicate ions of the active binder fraction is used as a hardener. The deactivation is to be caused by the cross-linkage and the resulting complete hardening of the used soluble glass binder system. This means that, for the regeneration of the sand, by use of an additional process step, a hardener for soluble glass is added, which leads to a largely completely chemical deactivation of the active alkaline binder remnants on the sand grain surface. This chemical deactivation takes place by a precipitation reaction of the soluble glass to silicic acid, in any case, to insoluble polysilicates, or corresponding to the used hardener, also to other reaction products, for example, zeolites.
The hardener may be an inorganic or organic acid or a salt. The inorganic acid may be a mineral acid, for example, hydrochloric acid or phosphoric acid, or carbon dioxide. As the organic acid, a mono-, di- or polycarboxylic acid can be used, such as citric acid or ethanoic acid. The salt may be an aluminate, for example, sodium aluminate; a sulfate, for example, aluminum sulfate; a phosphate, for example, aluminum phosphate; or a fluorosilicate, for example, fluorohexasilicate. It should be possible to use the hardener risk-free in an automated process. Furthermore, reaction products occurring during the deactivation of the active binder fraction should have no negative influence on the sand quality or the binder reaction when regenerated sand is used. The precipitation products should therefore preferably burn up in the thermal regeneration stage and no longer be contained in the salt.
The hardener can preferably be used as an aqueous solution. The addition of the hardener must not result in a clumping of the dry sand in the regeneration. Preferably, a fine wetting of the sand grain surfaces takes place by use of a solution of the hardener.
The sand mixed with the hardener is preferably moved in a fluid bed during the hardening reaction. As an alternative, a mixer can also be used. The hardening reaction can therefore take place during a defined reaction time. By use of the fluid bed, the sand can be moved along into a reaction chamber, in which the thermal reaction step is carried out. Instead of in a fluid bed, the sand can also be conveyed by way of a mechanical device, such as a screw conveyor.
The sand mixed with the hardener may also be intermediately stored before the thermal reaction step. For this purpose, a conveying of the sand takes place in an intermediate container and finally the thermal regeneration stage.
In the thermal regeneration stage, the deactivation of the binder, thus the hardening of the active binder to form an inactive binder, is completed.
For this purpose, the sand is heated to a temperature of preferably at least 200° C., particularly at least 500° C. The thermal regeneration stage can be carried out by way of a flame in order to deactivate the soluble glass remnants on the sand grains. However, the thermal regeneration stage can also be carried out in a fluid bed furnace to which the sand mixed with the hardener is fed.
In an advantageous embodiment of the invention, the mechanical regeneration stage comprises the following steps:
a) grain singling by the effect of mechanical crushers on the sand;
b) transferring the grain-singled sand into a pneumatic treatment chamber and moving the sand in the pneumatic treatment chamber in a fluid bed formed of air and sand; and
c) accelerating the sand in the pneumatic treatment chamber so that the singled sand grains rub against one another and the binder is at least partially removed from a grain surface.
As a result of the acceleration of the sand grains against the impact body, a particularly intensive load upon the grain surfaces of the sand grains, particularly an especially intensive rubbing of the sand grains against one another, can be achieved. It is thereby achieved that a large fraction of the binder remnants adhering to the grain surfaces is mechanically removed. The dissolved binder can then, for example, be removed together with a dust fraction from the pneumatic treatment chamber.
Advantageously, the hardener is added to blast air used for accelerating the sand. This permits a particularly rapid and intensive mixing of the hardener with the sand. However, it is also contemplated to add the hardener to the air used for producing the fluid bed.
According to a further development of the invention, the hardener will be added to the air and/or the blast air only after are predefined first treatment duration of the sand in the pneumatic treatment chamber. The first treatment duration is, for example, about 20 to 40 minutes, preferably about 25 to 35 minutes, counted from the start of the acceleration of the sand against the impact body. It was found that, after such a first treatment duration, a significant fraction of the binder adhering to the grain surfaces of the sand has been mechanically removed. Even in the case of longer treatment durations, the fraction of mechanically removable binder remnants will hardly increase. In order to keep the amount of hardener as low as possible, advantageously, the hardener will be used only when hardly any more binder remnants can be removed from the grain surfaces by the mechanical effect alone.
According to a particularly advantageous further development, liquid, particularly an aqueous hardener, is used. The liquid hardener is advantageously added in atomized form to the blast air and/or to the air. The atomization can take place in that the liquid hardener is atomized by use of a jet or an ultrasonic atomizer and is added to the blast air and/or to the air.
According to a further advantageous embodiment of the invention, new sand is added to the fluid bed after a predefined second treatment duration. The term “new sand” applies to sand which has previously not been mixed with a binder and used for producing sand cores or sand molds. The second treatment duration is advantageously selected such that it is longer than the first treatment duration. The addition of new sand can take place, for example, 20 to 60 minutes, preferably 45 to 55 minutes, after the start of the acceleration of the sand against the impact body. The added quantity of new sand corresponds to approximately 5 to 15%, preferably 7 to 13% of the quantity of sand to be regenerated. The new sand is advantageously added only when the hardener has had contact for a sufficiently long reaction time with the binder remnants remaining on the grain surfaces. The reaction time between the hardener and the sand to be regenerated amounts to 10 to 35 minutes, advantageously 15 to 30 minutes.
According to a further advantageous embodiment, the sand is heated in the fluid bed to a temperature of from 40 to 60° C., preferably 45 to 55°. As a result, particularly the reaction rate between the hardener and binder remnants remaining on the grain surfaces can be increased.
After passing through the mechanical regeneration stage, the mechanically regenerated sand will be present in pourable form. It is immediately thermally regenerated in the pourable form, as required, after an intermediate storage. This means that the mechanically regenerated sand is, in particular, not mixed with water, subjected to attritor grinding, wet grinding or the like as well as subsequent drying. The suggested method can therefore be carried out relatively rapidly and easily, in particular, also continuously.
According to a further advantageous development, the mechanically regenerated sand mixed with the hardener is transferred for the thermal regeneration into a furnace and is moved there at a temperature in the range of from 550° to 700° C. in an additional fluid bed formed of gas and the mechanically regenerated sand. The gas advantageously is a flammable gas. This means that the mechanically regenerated sand can therefore be moved in a flame. By heating the mechanically regenerated sand to a temperature of, for example, in the range of from 600° C. to 650° C., the hardened binder remnants still adhering to the grain surfaces are finally completely deactivated. Furthermore, the remnants of hardener are removed.
For saving energy, the mechanically regenerated sand can be preheated before being transferred into the furnace. For this purpose, warm waste air can be used that is discharged from the furnace.
Other objects, advantages and novel features of the present invention will become apparent from the following detailed description of one or more preferred embodiments when considered in conjunction with the accompanying drawing.